I'm hoping someone here can help me. I am terribly confused by the use of 'se' as a part of speech. I understand the addition of 'se' to the end of verbs to form reflexive verbs, and I understand 'sé' (both forms), but I don't at all understand how, why, or why to use 'se' in other contexts. For example:

Ellos se quedan aquí por la noche.Los niños se ríen.

Can someone explain to me how, why, and when to you use 'se'? I will be forever grateful!